Estimating Population

The world population has increased at a continuously compounded rate from 2.5 billion in 1950 to about 6.0 billion in 2002. Develop a mathematical model to forecast population in future years.
a) Graph the population against time and determine, identify or describe this relationship in your own words.
b) Find the average annual rate of change and use this rate to estimate the population in 2003.
c) When will the world population double to 12 billion and what is the average increase in people per year over this period?

I know that the formula is P=2.5e^(2.4)(t) and I know that the rate of change is 3.96 but I don't know how to answer the rest of the question. Please help I am so frustrated!!! Thanks